A GaAs single crystal wafer which is a material of GaAs (gallium arsenic) devices, is produced through steps such as cutting a GaAs single crystal ingot into wafers having a thickness of several hundred μm (slicing) and polishing. Si-doped n-type GaAs single crystal wafers doped with silicon (Si) as an impurity are used as substrates for lasers of DVD players and the like or substrates for light emitting devices such as light emitting diodes (LED).
Here, vertical boat methods such as the vertical gradient freeze method (VGF) and the vertical Bridgman method (VB) are known as methods for producing GaAs single crystal ingots. A vertical boat method is a method in which a seed crystal is placed at the bottom of a crucible, a material melt of a single crystal and a sealant in a liquid form is provided on the seed crystal, and the relevant crucible is cooled from a specific temperature profile, thereby growing a crystal upward from the bottom of the material melt. In the vertical gradient freeze method (VGF), the temperature itself is lowered, whereas in the vertical Bridgman method (VB), the crucible is relatively moved in a specific temperature profile. In producing a Si-doped GaAs single crystal ingot, Si is added into a GaAs melt.
In this method, in order, for example, to prevent As being a volatile component from evaporating from the ingot, B2O3 (boron oxide) is used as a sealant. In the crystal growth, at the interface between the GaAs melt and the liquid B2O3, an oxidation-reduction reaction of the following reaction equation is known to occur.3Si (in Melt)+2B2O3=3SiO2(into B2O3)+4B (in Melt)  (1)The balanced chemical equation (1) reaches a state of equilibrium at the start of the growth of a single crystal, and Si is considered to be concentrated into the GaAs melt due to segregation under normal freezing during the single crystal growth.
Here, if the reaction equation (1) proceeds to the right, Si in the GaAs melt decreases, whereas boron (B) is mixed from the liquid sealant into the GaAs melt. The mixing of boron should cause boron arsenide based contamination called scum, and for example, the scum would prevent GaAs from being crystallized into a single crystal, or would cause slip dislocations and threading dislocations, which results in lower quality of the GaAs single crystal.
JP 03-057079 A (PTL 1) discloses a technique of performing crystal growth with the equilibrium of the above balanced chemical equation (1) lying toward the left using a sealant of B2O3 into which silicon oxide is previously added. Meanwhile, JP 2000-109400 A (PTL 2) discloses a technique of stirring a liquid sealant during crystal growth in order to produce a Si-doped GaAs single crystal having uniform carrier concentration profile.